The following extended quotation from Montaigne biographer Sarah Bakewell is a perfect encapsulation of the classic trolley problem and the fat man variation:

You are walking near a trolley-car track when you notice five people tied to it in a row. The next instant, you see a trolley hurtling toward them, out of control. A signal lever is within your reach; if you pull it, you can divert the runaway trolley down a side track, saving the five — but killing another person, who is tied to that spur. What do you do? Most people say they would pull the lever: Better that one person should die instead of five.

Now, a different scenario. You are on a footbridge overlooking the track, where five people are tied down and the trolley is rushing toward them. There is no spur this time, but near you on the bridge is a chubby man. If you heave him over the side, he will fall on the track and his bulk will stop the trolley. He will die in the process. What do you do? (We presume your own body is too svelte to stop the trolley, should you be considering noble self-sacrifice.)

In numerical terms, the two situations are identical. A strict utilitarian, concerned only with the greatest happiness of the greatest number, would see no difference: In each case, one person dies to save five. Yet people seem to feel differently about the “Fat Man” case. The thought of seizing a random bystander, ignoring his screams, wrestling him to the railing and tumbling him over is too much. Surveys suggest that up to 90 percent of us would throw the lever in “Spur,” while a similar percentage think the Fat Man should not be thrown off the bridge. Yet, if asked, people find it hard to give logical reasons for this choice. Assaulting the Fat Man just feels wrong; our instincts cry out against it.

Sarah Bakewell: Clang Went the Trolley: 'Would You Kill the Fat Man?’ and ‘The Trolley Problem.’ New York Times Sunday Book Review  November. 22, 2013.

Printable pdf.


Self driving Lexus experimental prototype

Self driving Lexus experimental prototype

So, around 90% of folks would pull the lever; and similar high percentage would not push the fat man onto the track. This seems to hold true across age, gender and cultural background. So, what is going on? Ask students to articulate their own decisions with justifications for each of the two scenarios.

Students sometimes resist trolley problems as unrealistic text book cases. Point out to students that versions of the trolley problem presumably will have to programed in self-driving cars.  And what about the following scenarios:

  • When the atom bombs were dropped on Hiroshima and Nagasaki in 1945 it was stated that this was done to shorten the war and save lives. To what extent is this analogous to a trolley problem? Is it more like "the lever" or "the fat man"?

  • Can we say the same thing about the drone operator who may have to sacrifice bystanders in a market place in order to kill a suspected suicide bomber.

  • Finally, consider the following situation... What extra factors are involved?

This time you are to imagine yourself to be a surgeon, a truly great surgeon. Among other things you do, you transplant organs, and you are such a great surgeon that the organs you transplant always take. At the moment you have five patients who need organs. Two need one lung each, two need a kidney each, and the fifth needs a heart. If they do not get those organs today, they will all die; if you find organs for them today, you can transplant the organs and they will all live. But where to find the lungs, the kidneys, and the heart? The time is almost up when a report is brought to you that a young man who has just come into your clinic for his yearly check-up has exactly the right blood-type, and is in excellent health. Lo, you have a possible donor. All you need do is cut him up and distribute his parts among the five who need them. You ask, but he says, "Sorry. I deeply sympathize, but no."

Would it be morally permissible for you to operate anyway? Everybody to whom I have put this second hypothetical case says, No, it would not be morally permissible for you to proceed.

Judith Jarvis Thomson: The Trolley Problem. The Yale Law Journal, 94: 6 pp. 1395-1415. May, 1985.